Optical elements often require testing to determine optical and mechanical characteristics. For example, it is often necessary to test a lens for centration of one or both of a lens's surfaces.
Increasingly, lens designers have turned to aspherical surfaces to help control various types of optical aberrations that may occur in lenses having spherical surfaces. In general, an aspherical surface is considered to be shaped to a surface of revolution that is formed by rotating a non-circular curved shape about an axis of revolution. The surface of revolution is then rotationally symmetrical about the axis of revolution. Each aspherical surface that is a surface of revolution includes a vertex that is defined as the point on the surface where the surface intersects the axis of revolution.
Aspherical lenses provide various advantages over more spherical surfaces. For example, an aspheric lens may have a much shorter focal length than is possible with a spherical lens of the same diameter. This short focal length may be a useful feature where space is limited. A single aspherical lens may also be used as a condenser lens. In multilens systems, aspherics may help to correct aberrations.
Various improvements in lens design have also led to improvements in lens manufacturing as well. For example, injection molding of optical grade polymeric materials allows for the production of mass-produced high-quality optics that are made using lower-cost materials.
Plastic optics have a number of advantages over glass. Foremost of these are lower cost, higher impact resistance, lighter weight, and more configuration possibilities for simplifying system assembly. Configuration flexibility is especially useful in systems that use aspherical lenses to simplify system design and reduce parts count, weight, and cost. Moreover, light transmittance can be comparable to that of high-grade crown glasses. Finally, the plastics that can break generally do not splinter like glass. The fragments are larger and tend to be more obtuse and less hazardous.
Virtually all glass optic grinding and polishing equipment employs mechanisms that utilize mechanical movements for contouring spherical surfaces. Traditionally, finishing the optical pins of a mold for injection and compression-molding has been performed with a similar process. Hence, most optics produced have been spherical.
However, optical designers are using aspheres increasingly to reduce costs or to obtain performance unavailable using other techniques. Designs using aspheres often contain fewer elements. Further, the complex process of producing a precise aspherical mold cavity surface is required only once for each cavity. Consequently, the injection molding process is an economical technique for exploiting the advantages of aspheres.
However, various errors can occur during injection molding of aspherical lens surfaces that can produce surfaces that are decentered. For example, the optical axis of the surface may not be coincident with the mechanical axis of the lens. For example, the two mold pins that make up an injection mold may not be properly aligned with each other such that one or both lens surface's optical axes are not coincident with the mechanical axis of the lens. In addition, the curable material used in the injection molding process may unevenly shrink during curing.